1. Field of the Invention
The present invention relates to a spread spectrum communication apparatus and a telephone exchange system for performing communications through radio channels.
2. Description of the Prior Art
Conventionally, as a communication system capable of attaining a multiple access based on code division, a spread spectrum communication is known. In the spread spectrum communication, a transmitter converts a baseband signal to be transmitted into a baseband signal having a larger band width than that of original data based on a PN (Pseudo Noise) code having a sufficiently larger spectral width than that of the baseband signal to be transmitted. Furthermore, the transmitter modulates the converted signal based on, e.g., PSK (Phase Shift Keying) or FSK (Frequency Shift Keying) to form a high-frequency signal, and transmits the high-frequency signal.
However, the conventional spread spectrum communication suffers from a low data transmission speed since data is serially transmitted.
A conventional telephone exchange system performs an extension communication by radio based on FM (Frequency Modulation). Another conventional apparatus performs a data communication by radio based on PSK.
FIG. 21 shows the arrangement of a data conversion apparatus in a conventional telephone exchange system.
A case will be described below with reference to FIG. 21 wherein a conventional speech communication is performed. When speech data input from an exchange controller to a data conversion apparatus is to be transmitted, speech data, i.e., transmission baseband data 171 is mixed with an output signal from a transmission local oscillator 173 by a mixer 172 so as to be FM-modulated. The intermediate frequency of the output signal from the mixer 172 is amplified by an IF (Intermediate Frequency) amplifier 174, and the amplified signal is then filtered by a transmission filter 175. The filtered signal is input to a multiplexer 176, and is then transmitted from an antenna 177. A carrier wave upon modulation is assigned in units of 12.5 kHz, and an FM modulation factor is designed so that an occupied frequency after modulation does not exceed 12.5 kHz.
When an FM signal is to be received, an FM signal received at the antenna 177 is input to a mixer 179 through the multiplexer 176, and is mixed with an output signal from a reception local oscillator 178. Thereafter, the mixed signal is amplified by an IF amplifier 180, and the amplified signal is demodulated by a demodulator 181 into a baseband data output.
However, in the above-mentioned radio data communication, a high-speed data communication cannot be performed due to a problem of a limited use frequency band. More specifically, when there are a plurality of users, carrier waves of respective channels are set to have different frequencies to prevent interference among users, and at the same time, the use frequency band of each channel is limited. Therefore, when a high-speed data communication is performed, a spurious signal is generated. Thus, a simultaneous communication by a plurality of users suffers from a problem about frequency utilization.
When a very weak radiowave is utilized to solve the problem about frequency utilization, a transmission distance is undesirably shortened.